The invention will be described in the environment of digital video signal processing though it is to be understood to have much wider applications.
Signals, such as television signals, have a brightness component which must be referenced to a DC value to extract its information content. However, broadcasting such a signal tends to eliminate or lose the DC value. In order to reestablish a DC reference for the brightness component, television receivers are equipped with clamp circuitry. Typically this clamp circuitry is energized during the horizontal synchronization pulse, and responsive to the amplitude of this pulse sets the DC level for the remainder of the signal. This approach has proven to be quite satisfactory for most video signal processing applications.
It will readily be recognized by those skilled in the art of video signal processing that the amplitudes of horizontal sync pulses may vary from channel to channel. A consequence of this variation is that the absolute value of the clamped DC level will change from channel to channel.
The channel to channel variation is normally of little consequence in standard receivers. However in pix-in-pix receivers, which simultaneously display images from two channels in exclusive portions of the display screen, a difference in the brightness of the two images may be annoying to certain viewers. Therefore, it is desirable to be able to clamp the DC level of at least one of the signals to a specific value. In the instance of this invention it is the blanking level of the brightness signal. The blanking level of the brightness signal immediately precedes the active portion of a standard television signal and is relatively easily sampled.
It is an object of this invention to provide a blanking level clamp operative with binary or pulse code modulated (PCM) signals.